Only yield the exception to the block.

When the block is utilized, the developer probably has the url and the options
in the current scope and can use them without needing to pass them to the block.
fix markdown links
Fixing inconsistencies in Launchy#open, adding support for { :debug => true }.

Launchy#debug? and Launchy#dry_run? now always check environment variables and will now only return boolean true/false, not strings. Updated test cases to check for added consistencies.
Fixing typo: versino -> version.